Wordpress prompts "XML parsing error: XML or text declaration is not at the beginning of the entity"

Testing today wordpress It was found that ajax failed when adding categories. The browser f12 displayed the message "XML parsing error: XML or text declaration is not at the beginning of the entity". As shown below:

After searching on the Internet, I finally found out that the prompt appeared because there were redundant spaces on the page.

The debugger also confirms that this is the cause:

Once you know the cause of the problem, you can solve it through troubleshooting.

First, check the theme and change the default theme. It is proved that the theme is the cause.

Then check the function file that comes with the theme, and finally find that it is caused by redundant spaces in the back door of a file "?>" of the theme

If you happen to encounter this problem, you can check the following according to the idea of this article. Generally, it is caused by redundant spaces in the code of the theme or plug-in.